Apparatus for oxygen determination

ABSTRACT

APPARATUS FOR MEASURING OXYGEN CONTENT OF FLUIDS AT ELEVATED TEMPERATURES BY MEANS OF A GALVANIC CELL WITH A SOLID OXIDE ELECTROLYTE AND A REFERENCE ELECTRODE OF A MIXTURE OF A METAL AND ITS OXIDE INCLUDES A SENSOR WHICH COMPRISES A REFRACTORY BODY MEMBER HAVING A FIRST LONGITUDINAL OPENINGS FOR RECEIVING A FUSED SILICA TUBE WHICH HAS THE SOLID ELECTROLYTE FUSED TO THE END THEREOF WHICH CONTACTS THE FLUID. A ROD OF MOLYBDENUM OR OTHER CONDUCTOR EXTENDS THROUGH THE TUBE WITH THE REFERENCE ELECTRODE BETWEEN THE END OF THE ROD AND THE SOLID ELECTROLYTE. A SECOND FUSED SILICA TUBE EXTENDS THROUGH A SECOND LONGITUDINAL OPENINGS ADJACENT THE FIRST OPENING AND CONTAINS A SECOND ROD OF THE SAME MATERIAL AS THE FIRST. THE SECOND ROD ALSO HAS ITS FIRST END EXTENDING INTO THE FLUID. A THERMOCOUPLE EXTENDS THROUGH THE BODY MEMBER WITH ITS FIRST END EXTENDING INTO THE FLUID AND ITS SECOND END CONNECTED TO A CONNECTOR. A STEEL LANCE HAS MEANS AT ONE END FOR RECEIVING THE CONNECTOR AND SECOND ENDS OF THE RODS, AND MEANS AT THE OTHER END FOR CONNECTING TO A VOLTAGE MEASURING MEANS. IN SOME INSTANCES THE THERMOCCOUPLE MAY BE OMITTED.   D R A W I N G

Sept- 11, 1973 R. s. RITTIGER ETAL 3,758,397

APPARATUS FOR OXYGEN DETERMINATION Filed July l5, 1970 2 Sheets-Sheet lvF/at f Afro/'nay APPARATUS FOR OXYGEN DETERMINATION A/lorney UnitedStates Patent O APPARATUS FOR OXYGEN DETERMINATION Robert S. Rittiger,Allegheny Township, Westmoreland County, and Charles K. Russell,Franklin Township,

Westmoreland County, Pa., assignors to United States Steel CorporationFiled July 15, 1970, Ser. No. 54,986 Int. Cl. G01n 27/46 U.S. Cl.204-195 S 4 Claims ABSTRACT OF THE DISCLOSURE Apparatus for measuringoxygen content of uids at elevated temperatures by means of a galvaniccell with a solid oxide electrolyte and a reference electrode of amixture of a metal and its oxide includes a sensor which comprises arefractory body member having a rst longitudinal opening for receiving afused silica tube which has the solid electrolyte fused to the endthereof which contacts the fluid. A rod of molybdenum or other conductorextends through the tube with the reference electrode between the end ofthe rod and the solid electrolyte. A second fused silica tube extendsthrough a second longitudinal opening adjacent the first opening andcontains a second rod of the same material as the tirst. The second rodalso has its first end extending into the fluid. A thermocouple extendsthrough the body member with its first end extending into the fluid andits second end connected to a connector. A steel lance has means at oneend for receiving the connector and second ends of the rods and means atthe other end for connecting to a voltage measuring means. In someinstances the thermocouple maybe omitted.

This invention relates to apparatus for measuring the oxygen contentand/or activity of fluids at elevated temperatures, and moreparticularly, to apparatus that rapidly determines the oxygen content ofliquid steel in a furnace or container without removing a sample. Sinceour invention, at present, is most useful and most needed for thispurpose, this use will be stressed hereinafter. However, our inventionis also applicable for determining oxygen in other fluids attemperatures above approximately 700 C., such as liquid copper and hotfurnace gases.

Apparatus suitable for this purpose is disclosed in the copendingapplication of Fruehan and Turkdogan, Ser. No. 39,530, filed May 21,1970. However, the apparatus disclosed therein is basically laboratoryapparatus and is not suitable for use in large steelmaking furnaces andcontainers because of cost, fragility, reliability when used byrelatively unskilled workmen, ease of use and other factors important inadapting the basic cell concept to industrial use.

It is therefore an object of our invention to provide apparatus formeasuring oxygen content and temperature in substantially the sametheoretical manner as that of the Fruehan et al. application, but whichis cheaper to manufacture, more rugged and which overcomes the otherdisadvantages of the Fruehan et al. device.

This and other objects will be more apparent after referring to thefollowing specification and attached drawings, in which:

FIG. 1 is a view, mostly in longitudinal section of one embodiment ofour invention;

FIG. 2 is a sectional view, similar to FIG. l, showing a secondembodiment of our invention, but with the upper part of the apparatusand the connections to the potentiometer omitted;

FIG. 3 is a view taken on line III- III of FIG. 2 with parts omitted;

FIG. 4 is a viewtaken on line IV-IV of FIG. 2; and

FIG. 5 is a sectional view of the lower part of a third sensor of ourinvention.

ICC

Referring more particularly to FIG. 1 of the drawings, reference numeral2 indicates the sensor unit of our apparatus. This includes a bodymember 4 which is made of a heat and thermal-shock resistant refractorysuch as cast aluminum oxide. A iirst longitudinal opening 6 extendsthrough the body member 4 and contains a first tube 8 whose first endextends into the molten steel or other lluicl L. The tube 8 is made ofan electrical insulator such as fused silica. A solid electrolyte in theform of a disc 10 of ZrO2 containing 3 to 10% by weight of CaO is fusedto the inside of tube 8, and a reference electnode 12 in the form of amixture of powdered Cr and Cr203 is placed on top of the electrolyte 10in contact with the first end of a rod 14 which extends through tube 8beyond the second end of body member 4. The rod 14 is made of anelectrical conductor, preferably molybdenum. A second longitudinalopening 16 extends through the body member 4 adjacent to and generallyparallel to opening 6. A tube 18, similar to tube 8, extends throughopening 16 and receives a rod 20 of the same material as rod 14. Theperiphery of disc 10 is covered with a refractory coating 22 and theperiphery of the first end of rod 20 is covered with a similar coating24 to prevent too rapid disintegration of the disc and rod. The ends ofthe disc 10 and rod 20 are free of the coating which is preferably ofthe same composition as body member 4.

A third longitudinal opening 26 is provided adjacent to and generallyparallel to the openings 6 and 16, but does not extend the full lengthof the body member 4. A closed end tube 28, made of insulating materialssuch as fused silica or quartz tits within the opening 26 and receives a2-hole refractory protection tube 30 for the wires of a thermocouple 32which is preferably a standard Pt, Pt+10% Rh thermocuple when used withmolten steel. The exposed upper ends of the thermocouple wires arecoated with an epoxy or other impervious electrically insulating cementand are crimped into suitable thermocouple terminals 34. The second ortop ends of rods 14 and 20 also act as terminals 14T and 20T.

The sensor is preferably made as follows: The disc 10 is placed insideof tube 8 which is heated to a temperature at which it softens and flowsaround the disc 10. Upon cooling, the fused silica of tube 8 forms atight seal around the disc 10 so as to prevent leakage of steel duringimmersion. If desired, a small diameter tube with a closed end made ofthe electrolyte material may be substituted for the tube 8 and disc 10.The mixture 12 is then tamped into the tube 8 and the rod 14 insertedinto the tube in contact with the mixture 12. 'Ihe rod 20 is placed intube 16 and the parts 28 to 34 assembled into one unit. All three tubeassemblies are then placed in a mold cavity and held in essentiallyparallel relationship with each other and the axis of the mold cavity bymeans of a suitable fixture. A slurry of refractory particles andbonding agents mixed with Water is then poured into the cavity andaround the stated components until the mold cavity is filled. Each ofthe component ends that must contact the molten steel projectapproximately 3A inch beyond one end of the mold and are not covered bythe slurry. Also, the other end of the components project beyond theopposite end of the mold and are not covered by the slurry. A smallquantity of this slurry is painted onto the sides of the electrolytedisc 10 and the end of tube 8. However, the end surface of theelectrolyte disc is not covered with slurry. Also, the exposed length ofrod 20 that contacts molten steel is covered with refractory slurryexcept for its end.

After the bonding agents in the slurry form a hard compact mass with therefractory grains, the oxygen sensor unit is removed from the moldcavity and is subjected to a series of drying steps that will render therefractory body even stronger and free of moisture. lt is preferred touse a commercial refractory castable, such as Castola'st G, manufacturedby the Harbison-Wa'lker Refractory Company, for this purpose and tofollow their recommended for-ming and drying procedures.

It is preferred to cover the ends of the components of the sensor 2extending below the body member 4 with steel caps 40 and 42 with cap 42extending around cap 40 and around the end of body member 4. Thisprotects the electrolyte from slag contamination during immersion andthe entire sensor from physical damage during handling. Steel ispreferred to Ibras's or aluminum since it gives better physicalprotection prior to use, and because of its higher melting point,permits a desirable delay in melting time during immersion.

A steel lance 44 has a receptacle 46 at its lower end for receivingterminals 14T, 20T and 34, which are conneoted to a millivolt recorder48, preferably a two-pen potentiometric Type-82 by means of a suitabletwoconductor compensating lead wire 50 for the ther-mocouple and atwo-conductor lead wire 52 for the oxygen cell. The length of lance 44may be varied as desired and could also be omitted. Cardboard or ceramicsleeves 54 and 56 are preferably provided to protect the parts fromoverheating during immersion.

In operation, the assembly is plunged into the melt to a suitable depth.The sleeves 54 and 56, which extend above the top of the melt, protectthe Wires and upper elements of the sensor from damage by the metal.Caps 40 and 42 protect the sensor from chemical attack by the slag as itis lowered through the slag into the molten metal. When the stabilizedzirconium oxide electrolyte contacts the electrode of liquid steel, agalvanic cell results which may be represented as follows:

CaO are in solid form and Fe-O is liquid).

The voltage across the electrolyte is given by the free energy equationas follows:

[percent O] log Kpozl/Z E=aso3 (1) where The temperature dependence ofthe equilibrium iconstant K and p= for Cr-Cr203 equilibrium from knownthermodynamic data is plunging the sensor into the melt fand providescontinuous oxygen content readings up to 40 minutes.

By combining these equations and substituting values of the constants,the following equation is obtained for weight percent oxygen in liquidsteel in terms of ELMF (E) in rnillivolts and T in degrees Kelvin forthe Cr-CR2O3 reference electrode and receives a rod 72 of While thespecific components set forth above are preferred since -they haveproven very sucessful in use in determining the oxygen content of liquidsteel, they may be varied ,as long as they meet the followingrequirements. The electrolyte 10 must be a solid oxide and an oxygen ionconductor with insignificant electronic conductivity under theconditions of its use, particularly at the temperature and oxygenpartial pressure to which it is subjected. It must not react with thematerials it contacts. ZrOlg with 3 to 10% by weight of OaOf or MgO;ThO2 with 3 to 20% by weight of YgO, A1203 or MgO are examples ofmaterials which are suitable under vlarying conditions, but allelectrolytes are not all suitable for use with all types of fluids.

The reference electrode 12 must be a condensed phase mixture of la metaland its oxide which does not readily melt at the temperature to which itis subjected no1 react significantly with the materials itv contacts.Cr-Cr2O'3 is most suitable for use with steel. MolMoOg, Ta-TaO2, W-Wo2,Ni-NiO, Fe-FeO, and (Eo-C00, may also be used, but not all can be usedwith all types of iiuids. In place of Mo the conductor rods 14 and 20may be made of Pt, Rh, Pt and Rh, W, Ta, W and Re, Cr :and alloysthereof. Ni, Co, Fe, Cu, Al, Sn, Zn, Mg and their alloys may be used fortesting most nonferrous metals including Al, Zn, Cu, Cr, Ni, Co and Snand for testing most gases. However, it should be understood that it isthe mechanical arrangement of the parts, nather than the specificmaterials used, that is the important feature of the invention.

FIGS. 2, 3 and 4 show the preferred embodiment of our invention. Thisincludes a body member 58 preferably made of the same material as bodymember 4 of the first embodiment. A rst longitudinal opening 60 extendsthrough the body member and contains a first tube 62 whose rst endextends into the molten steel or other fluid L. The tube 62 is made ofan electrical insulator such as fused quartz. A solid electrolyte in theform of a disc 63 of ZrOlz containing 3 to 101% by weight of CaO isfused to the inside of tube 62, and a reference electrode 64 in the formof a mixture of powdered Cr and Cr2O3 is placed on top of theelectrolyte 63 and held in position by a retainer washer 65 which may bea refractory. A rod 66 in tube 62 has its first end bent and its secondend extending through the tube 62 lbeyond the second end of body member58. The rod 66 is made of `an electrical conductor, preferablymolybdenum. The washer 65 has an opening 68 therethrough for receivingthe bent end of rod 66 which extends longitudinal opening 70 ber 58adjacent to and into the electrode 64. A second extends through the bodymemgenerally parallel to opening 60 the same material as rod 66.

The exposed lower end of tube 62, the periphery of disc 68 and theexposed lower end of rod 72 may be covered with a refractory coatingexcept for a short portion of the end of disc 63 and the end of rod 72.

Third and fourth longitudinal openings 74 and 76 generally parallel toopenings 60 and 70 receive a U-tube 78 made of fused quartz or otherinsulating material. Thermocouple wires 84 and 82 are received in theU-tube 78 and joined at the bend of the tube. This construction isessentially that shown in Mead Pat. No. 2,999,121. A body cap 84 ofpolyethylene or other electrically insulating material abuts the secondend of body member 58 and also receives a polyethylene connector tube86. Rod 66 passes through tube 86 with its top end bent around thebottom of a slot S8 in the tube 86 and back down the outside of thetube. Rod 72 likewise passes through tube 86 with its end bent aroundthe free or top end of the tube. Thermocouple wire is connected to aconductor 90 which passes through tube 86 with its topend bent aroundthe bottom of a slot 92 in the tube 86 to the outside of the tube.Thermocouple wire 82 is connected to a conductor 94 and passes throughthe tube 86 with its end bent around the bottom of a slot 96 in the tube86 to the outside of the tube. The upper ends of Wires 80 and 82 arecoated with an epoxy as in the first embodiment. A receptable 9S ofinsulating material, such as rubber, having four spaced apart electricalcontact rings 98a, 98b, 98e and 98d therein surrounds the tube 86 withthe rings 98a, 98b, 98C, and 98d in contact with conductors 90, 94, 66and 72, respectively. Four terminals 100:1, 100b, 100C and 100d securedin the end of receptacle 98 are electrically connected to rings 98a,98b, 98e` and 98d, respectively. A receptacle adapter 102 made of steelor black iron pipe surrounds receptacle 98 and is connected by means ofthreaded insert 104 to a lance 106 corresponding to lance 44 of thefirst embodiment. A cardboard sleeve 108 surrounds the body member 58,body cap 84 and tubes 102 and 106. A steel protection cap 110 having asmall hole 112 in the end thereof is secured to the sleeve 108. Theconnections of the terminals to the potentiometer may be as in the firstembodiment and the operation is the same as that of the firstembodiment. This embodiment is made in generally the same manner as thefirst embodiment.

FIG. 5 shows a simplified and less expensive embodiment of our inventionin which the thermocouple is omitted. Except for this omission theconstruction and materials are essentially the same as those of theother embodiments. This may be done because in many operations,particularly in steel making, other means are provided for continuouslyor intermittently measuring the temperature of the fluid and thetemperature so obtained can be utilized in combination with the oxygensensor to obtain an accurate oxygen reading. This embodiment includes abody member 114 made of the same material as body member 4. A firstlongitudinal opening 116 extends through the body member 114 andcontains a silica tube 118 whose first end extends into the molten steelor other fiuid L. A solid electrolyte in the form of a disc 120 is fusedto the inside of tube 118, and a reference electrode 122 in the form ofa mixture of powdered Cr and Cr203 is placed on top of the electrolyte120 in contact with the first end of a conductor rod 124 which extendsthrough tube 118 beyond the second end of body member 114. A secondlongitudinal opening 126 extends through the body member 114 adjacent toand generally parallel to opening 116. A tube 128, similar to tube 118,may extend through opening 126 to receive a rod 130 of the same materialas rod '124. If desired, the tube 118 may be omitted and the rod 130received directly in opening 116. The remaining part of the sensor maybe as shown in either of the other embodiments with the thermocoupleconnections omitted and hence are not shown or described here except toshow a cardboard tube 132 and steel cap 134.

While several embodiments of our invention have been shown anddescribed, it will be apparent that other adaptations and modificationsmay be made without departing from the scope of the following claims.

We claim:

1. In apparatus for determining the oxygen content of a liquid metalwhich comprises a galvanie cell including said liquid metal as anelectrode, a solid oxide electrolyte and a reference electrode, saidelectrolyte being an oxygen ion conductor having insignificantelectronic conductivity under conditions of use, said referenceelectrode being a powdery mixture of a metal and its oxide, and meansfor measuring the EMF of the cell when the electrolyte contacts theliquid metal, the improvement comprising a sensor; said sensor includingsaid electrolyte and said reference electrode, an elongated heat andthermal shock resistant body member of cast alumina adapted to bepartially inserted into said liquid metal and having a plurality oflongitudinal openings therein, a first fused silica tube extendingthrough the first of said openings beyond both ends of said body member,a disc of said electrolyte fused to the inside of one end of said firsttube spaced away from said body member and adapted to contact saidliquid metal, means for containing said powdery mixture of a metal andits oxide Within said tube and in contact with said electrolyte, a firstconductor extending through the first tube thereby insulating said firstconductor from said body member and having a first end contacting saidpowdery mixture of a metal and its oxide, a second conductor extendingthrough the second of said openings beyond both ends of said body memberwith the first end of said second conductor adapted to contact saidliquid metal near said electrolyte, said conductors being of the samematerial, terminal means connecting the second ends of said conductorsto said voltage measuring means, a thermocouple comprising twodissimilar wires extending through said body member with the joined endsof said wires extending beyond the first end of said body member and theother ends connected to said terminal means said thermocouple adapted tocontact said liquid metal near said electrolyte, and means forinsulating said thermocouple Wires from one another, from saidconductors and from said body members.

2. Apparatus according to claim 1 which includes a lance having means atone end for receiving said terminal means and means at its other endconnected to said voltage measuring means, a cardboard sleevesurrounding at least a portion of said lance and said body member, and asteel cap surrounding the first end of said conductors and saidthermocouple.

3. Apparatus according to claim 1 in which said conductors aremolybdenum rods, said electrolyte is ZrOZ containing 3 to 10% by weightCaO and said reference electrode is a mixture of powdered Cr and Cr203.

4. Apparatus according to claim 2 in which said conductors aremolybdenum rods, said electrolyte is ZrOZ containing 3 to 10% by WeightCaO and said reference electrode is a mixture of powdered Cr and Cr203.

References Cited UNITED STATES PATENTS 3,454,486 7/1969 Davies 204-1953,464,008 8/1969 Meysson et al. 204-195 3,468,780 9/ 1969 Fischer204-195 3,481,855 12/ 1969 -Kolodney et al 204-195 3,616,407 10/1971lEngell et al. 204-195 S 3,619,381 11/1971 Fitterer 204-195 S FOREIGNPATENTS 1,191,222 5/ 1970 Great Britain 204-195 OTHER REFERENCES Wilder,Trans of the Metallurgical Soc. of AIME, v01. 236 July 1966, pp.1035-1040.

Fitterer, Reprint from Journal of Metals, August 1966, pp. 1-6.

TA-HSUNG TUNG, Primary Examiner

